Actually, they don't need to be opensource. Having free firmware or being a simple machine to obey is enough. There are good printers listed on h-node. People mock fsf ryf certification but I think it is very much valid.
What good is free software when manufacturers can just require the firmware to be signed, thereby disallowing any and all modifications? We'll never be truly free until we can manufacture this hardware at home, just like we can make our own software at home.
Kragen is right that we need hackers working on building up our hardware capabilities in general. The free software movement started in an era of relatively open hardware when that seemed less of a priority.
Admittedly I haven't done anything myself, though I admire those who have.
wanting access to the source code of an early Xerox laser printer (at MIT) is what got Richard Stallman ticked off enough to devote his life to free software.
when I saw the headline for this article I first read it as "Canon sued for disabling scammer when printers run out of ink" :)